Solid-state imaging systems or imaging readers have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one- and two-dimensional bar code symbols, each bearing elements, e.g., bars and spaces, of different widths and reflectivities, to be decoded, as well as non-symbol targets or forms, such as documents, labels, receipts, signatures, drivers' licenses, employee badges, and payment/loyalty cards, each bearing alphanumeric characters and graphics, to be imaged. A known exemplary imaging reader includes a housing either held by a user and/or supported on a support surface, a window supported by the housing and aimed at the target, and a scan engine or imaging module supported by the housing and having a solid-state imager (or image sensor) with a sensor array of photocells or light sensors (also known as pixels), and an imaging lens assembly for capturing return light scattered and/or reflected from the target being imaged through the window over an imaging field of view, and for projecting the return light onto the image sensor to initiate capture of an image of the target over a range of working distances in which the target can be read. Such an image sensor may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing and processing electrical signals corresponding to a one- or two-dimensional array of pixel data over the field of view. These electrical signals are decoded and/or processed by a programmed microprocessor or controller into information related to the target being read, e.g., decoded data indicative of a symbol target, or into a picture of a non-symbol target.
In order to increase the amount of the return light captured by the sensor array, especially in dimly lit environments and/or at far range reading, the imaging module generally also includes an illuminating light assembly for illuminating the target with illumination light for reflection and scattering therefrom. Since the operator of the imaging reader cannot see exactly whether a target is located entirely within the illuminated field of view of the sensor array, or know whether the target is optimally centrally located within the illuminated field of view, the imaging module also typically includes an aiming light assembly for projecting a visible aiming light pattern, for example, a generally circular aiming spot, or an aiming cross-hairs, for placement at or near the center of the target, or an aiming line, or a series of generally circular aiming spots linearly spaced apart, for placement lengthwise along the target, to assist the user in visually locating the target within the imaging field of view and, thus, advise the user in which direction the reader is to be moved in order to accurately position the aiming light pattern on the target prior to reading. The aiming light assembly includes at least one aiming light source, such as a laser or a light emitting diode (LED), an aiming lens, a field stop, and, sometimes, a pattern shaping optical element, such as a diffractive optical element (DOE), or a refractive optical element (ROE).
As advantageous as such known aiming light assemblies have been, they have proven to be less than satisfactory in certain situations. For example, many compact aiming light assemblies often generate a generally circular aiming spot to mark the center of the imaging field of view. A very compact and power efficient design to do so typically consists of a laser for directing a laser beam through a collimating lens. However, some users, particularly working in the healthcare and retail fields, do not wish to emit laser beams from their imaging readers, primarily out of unwarranted safety concerns for patients and customers, and instead, prefer to use the non-laser light beam emitted from an LED to form the aiming light spot.
However, there are problems with using an LED as the aiming light source. The LED emits its aiming light beam with a large divergence angle. Also, the LED has irregularities over its LED chip surface. These irregularities are caused by the presence of contacts, wires, etc. on the chip surface, and are visible as optical artifacts in the aiming light spot, thereby making the aiming light spot irregular in appearance. The large divergence angle and these optical artifacts make it difficult to design a compact, aiming light assembly capable of optically forming a small, uniform, bright aiming light spot over an extended range of working distances without losing much of the optical output power of the LED.
Thus, as shown in the ray diagram of FIG. 7A, the prior art has proposed creating an aiming spot by locating an LED 1 (extending between points A1 and A2) at a distance X behind a field stop 2 (extending between points B1 and B2), and then imaging the field stop 2, with the help of an aiming lens 3 (extending between points C1, C2, C3 and C4), to a certain distance D2 relative to the aiming lens 3 within the working distance range of the imaging reader. Points B1′ and B2′ are images of the points B1 and B2 of the field stop 2 via points C1 and C4 on the aiming lens 3. Points A1′ and A2′ are images of the points A1 and A2 of the LED 1 via points C2 and C3 on the aiming lens 3, and are located at a certain distance D1 relative to the aiming lens 3 within the working distance range of the imaging reader. In FIG. 7A, the LED 1, the field stop 2, and the aiming lens 3 are all symmetrically located on an optical axis 4 of the aiming lens 3. The aiming lens 3 has a lens aperture 5 through which the aiming light passes.
The distance X is typically set to be relatively large in order to, among other things, achieve uniformity of the aiming spot and, as explained below with reference to FIG. 7B, to tolerate any offset or misalignment between the LED 1 and the field stop 2. For example, the aforementioned optical artifacts are visible in the aiming spot at the distance D1 and, in order to prevent such artifacts from interfering with efficient aiming in the working distance range, the distance D1 should be decreased, which, in turn, dictates that the distance X should be increased. As another example, some LEDs have integrated dome lenses to reduce the divergence of the emitted aiming light beam, in which case, the apparent position of the LED 1 moves to the left in FIG. 7A and, in other words, the distance X effectively increases. However, an increased distance X results in high optical losses and low optical power for the aiming light spot. A dim aiming light spot is not readily visible, especially in the far working distance range, thereby degrading and compromising the entire aiming process. Also, an increased distance X increases the overall size of the aiming light assembly and the imaging module, thereby making it difficult to accommodate miniature, compact imaging readers.
As previously mentioned, if there is an offset between the LED 1 and the field stop 2, as shown in the prior art ray diagram of FIG. 7B in which the LED 1 (having point A3 between points A1 and A2) is asymmetrically located relative to the optical axis 4, then the aiming light spot becomes non-uniform in brightness due to vignetting of the aiming light beam on the aiming lens 3. Using the same reference characters as in FIG. 7A, point B2′ of the aiming light spot is an image of the point B2 of the field stop 2. Point B2′ collects light only from the fractional portion A2-A3 of the LED 1, because the light ray (shown in broken lines between points A1 and B2) misses the aiming lens 3. At the same time, point B1′ of the aiming light spot collects light from the entire LED 1, thereby making point B1′ much brighter than point B2′. To make the aiming light spot uniform in brightness, the aiming lens 3 would need to be enlarged, thereby rendering the aiming light assembly less compact. Alternatively, the distance X would need to be enlarged, which, as explained above, would further cause additional power losses.
Accordingly, there is a need to increase the brightness and the uniformity of the aiming light spot generated by an LED-based aiming light assembly in an imaging reader, without increasing the overall size of the aiming light assembly and of the imaging module, and with improved tolerance to offsets or misalignment between components of the aiming light assembly, over an extended range of working distances.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.